Agricultural tractors are generally provided with at least one power take off shaft in order that implements, such as rotary harrows, fertilizer spreaders, sprayer pumps, balers, etc may be driven.
Implement manufacturers now manufacture tractors and implements to run at one of three standard speeds, namely, 540 rpm, 1000 rpm, and ground speed. The term “ground speed” means that the rate of rotation of the power take off shaft is proportional to the speed at which the tractor travels across the ground.
Since implements are manufactured to these standards, it is necessary for tractors to be provided with power take off shafts which rotate at the required speed. Power take off gear trains have been developed which provide for the selection of different speeds, eg 1000 rpm, 540 rpm, ground speed. Clearly, in the case of a fixed rotation speed, i.e. 1000 rpm, or 540 rpm, this can only be achieved at a certain engine speed. The manufacturer selects this speed, and usually a speed towards the maximum engine speed is selected, thereby permitting engine speed to fall as load on the power take off shaft increases.
Where the load on the power take off shaft is relatively low, the torque back up provided by running the engine at a high speed is not required. It is therefore not efficient to run the engine at high speed, but this is required in order for the power take off shaft to rotate at the required speed. In order to overcome this problem, manufacturers often provide what is known as a 540 rpm economy power take off shaft. Effectively, the gear train is arranged such that the power take off shaft runs at 540 rpm with an engine speed of about two thirds maximum speed, and as such the tractor's fuel economy improves and the engine is quieter. With further gearing, an economy mode at 1000 rpm may also be provided.
Problems can arise, however, with providing an economy mode in that the additional gearing takes up valuable space in the already crowded area of a tractor rear axle, and also it is not generally possible to shift from economy to conventional mode (without stopping the tractor) if there is a sudden torque demand.